Exoplanets.
They’re just like regular planets but found in other planetary systems besides our own.
So far over 3,600 discoveries have been confirmed since 1992.
These fuzzy dots are a few examples of the best direct images that we’ve got
and these are all gas giants like Jupiter.
What we don’t have is a really good image of an Earth like exoplanet.
It may actually be possible to get an image like this, but how we could do it may surprise you
--we could use the sun as a lens.
The sun is massive, to say the least, therefore so is its gravitational effect
which warps the very fabric of space itself.
When incoming light from a hypothetical exoplanet approaches the sun, its path is also warped.
These curved light rays are brought into focus
starting from about 550 astronomical units, away from the Sun.
The effect of gravity on the deflection of light is inversely proportional to its distance
from the center of the sun.
Approaching light rays further from the sun are not curved as much as light rays closer
to it so they come into focus past 550 AU, which results in a focus line rather than
a single focus point.
This is the solar gravitational lens.
If we take a 1 meter telescope - place it at 650 AU away on the focus line -
targeting an exoplanet 100 light years away - how much magnification
and resolving power do you think it will have?
From a fuzzy dot to a slightly larger fuzzy dot?
Not even close.
It could resolve details at the scale of 10 kilometers squared.
That’s like resolving the width of a single human hair on the moon from Earth
or an equivalent resolution like this image of Earth.
If instead you targeted the closest exoplanet to us, proxima b at about four and a quarter
light years away, the resolution would be even greater—in the hundreds of meters scale.
But there are not as many planetary systems to choose from right next door to us
relatively speaking.
The sun, obviously, does not function exactly like a conventional lens.
The sun’s gravitational force warps the incoming light in addition to focusing it,
resulting in a ring shape around the sun, called an Einstein ring.
Correcting this is a lot more work than a de-warping filter in photoshop.
Adding to the difficulty, we can’t resolve the whole Einstein ring at once with just
a 1 meter telescope either.
If this same example exoplanet has an Earth like diameter of around 12,700 km then that
will result in an Einstein ring approximately 1.3 km thick.
This animation isn’t to scale as the ring would look more like
this compared to the sun.
But with those variables in mind, the area of the focus line that our telescope needs
to cover would be a cylinder with a diameter of about 1.3 km.
You would need a telescope that’s at least that size to resolve the entirety of the Einstein
ring in one picture.
That’s 260 times larger than the primary mirror in the hubble telescope.
Fortunately, you don’t need to resolve the whole ring in just one picture.
Our 1 meter telescope can image an area on this example exoplanet 10 kilometers squared.
So you can think of each picture it takes as a single pixel.
You can still resolve the whole ring, you just need to assemble it pixel by pixel.
The proposed goal is for a final image with 1000 by 1000 pixels.
But that’ll take some times as that adds up to a total of one million pictures.
Before the imaging process can even begin, something has to be done about the sun.
The telescope needs to face the sun to image the exoplanet but, unsurprisingly, its light
would outshine the exoplanet.
Since the Einstein ring is around and outside the sun, an internal coronagraph can be used
which blocks the sun and the brightest part of the solar corona.
There will still be some light from the corona mixed in but not enough to completely overwhelm
the exoplanet’s light.
For the coronagraph to be more effective the telescope needs to be positioned further back
on the focus line, that’s why we can’t place the telescope right at 550 AU, but still
not too far back as that could add years to the mission timeline.
A good compromise would be between 650 to 800 AU.
When the telescope is further from the sun, naturally it will appear smaller.
The magnification of the exoplanet stays the same but the Einstein ring is now at a greater
distance from the sun’s surface, so there’s less coronal light to contend with.
But how far away are these distances really?
An astronomical unit, or AU, is the distance from the Earth to the Sun.
At its greatest distance from the sun, Pluto, is almost 50 AU away.
Voyager 1 is currently travelling through interstellar space at 138 AU away from the
sun; farther than any spacecraft has travelled.
The telescope needs to be almost 5 times further than that.
That distance, while considerable, may not be insurmountable.
Here’s one possible, hypothetical scenario.
Using the currently in development SLS rocket as the launch vehicle, the spacecraft can
get to Jupiter within six months.
It can use a gravity assist at Jupiter, slingshotting the spacecraft towards the sun.
As it falls into the sun’s gravity well, within 5-7 solar radii
its velocity dramatically increases.
When the spacecraft reaches maximum velocity through this maneuver, its rocket engines
fire, adding to its acceleration and sending it on a trajectory to the focus line.
Travelling between 17-22 AU per year the spacecraft can get to 650 AU in about 30 years in the
most optimistic scenario.
But no spacecraft has approached this close to the sun before, so a fairly robust solar
heat shield will have to be designed and employed in order for the spacecraft to survive.
Once there, the telescope has to move with the focus line and within it as nothing in
the universe is static.
The exoplanet orbits its parent star while also rotating around its axis—if not tidally
locked to its own star.
While the trajectory design can account for much of this, a novel design for the spacecraft
itself is still needed for it to move stably within the focus line.
One idea is to build a spacecraft with the telescope on one end tethered to a counter
weight at the other end using ion thrusters for propulsion at this point.
The telescope can be pulled in or extended out along this tether to move within the focus
line while maintaining a stabilized anchored position.
This enables the telescope to gather a sufficient amount of images with less difficulty than
keeping an untethered spacecraft stabilized.
Using this method it should take around 3 months to finish the task.
But the challenges don’t end there.
Each image that the telescope takes is not a neat slice of the full ring.
Instead the telescope builds a rasterized image, where each snapshot contributes more
detail and magnification of a specific area on the exoplanet.
These images end up overlapping each other, which can be considered a benefit as we won’t
need quite as many for the desired resolution.
Next, a deconvolution algorithm will be needed to fix the warped ring.
While that may be a considerable challenge, we’ll have the variables we need to correct
it; the position of the spacecraft; brightness of each image at those positions and the optical
properties of the solar gravitational lens.
If we are able to overcome all of these technical hurdles we will finally have our first high
resolution image of an exoplanet.
Using spectroscopy, we can analyze the light from the exoplanet
in more detail than ever before.
Gases absorb and emit their own distinct wavelengths of light.
So when we analyze light from its atmosphere we’ll be able to accurately define its composition.
Is the air breathable?
Are there any telltale signs of life in the atmosphere like methane for example?
Suppose there’s actually intelligent life, and they too have electricity.
Well if it’s night time and they turn on the lights, we’ll see them.
But there’s more.
Radio waves are just another wavelength of light.
If ET is broadcasting, those radio transmissions will also be magnified, but not to the extent
of visible light as the radio spectrum is actually distorted
by the interference of the sun’s corona.
While utilizing the Solar Gravitational lens is a daunting technical challenge, it still
may be achievable in the near future.
In fact, NASA’s Innovative Advanced Concepts programs, or NIAC, recently accepted a proposal,
lead by Dr. Slava Turyshev to further explore this very concept.
This video is based largely on his paper and follow up discussions with him.
If you want to know more you can find a link to his paper in the description below
as well as other relevant resources.